1. Field of the Invention
The present invention relates to a band-pass filter element and to a high frequency module incorporating the band-pass filter element and a layered substrate.
2. Description of the Related Art
Recently, cellular phones operable in a plurality of frequency bands (multibands) have been put to practical use. The third-generation cellular phones having a high-rate data communication function have also been widely used. It is therefore required that cellular phones be operable in multiple modes and multiple bands.
For example, cellular phones that conform to the time division multiple access system and that are operable in multibands have been practically utilized while cellular phones that conform to the wide-band code division multiple access (WCDMA) system have been practically utilized, too. To make communications through the WCDMA system accessible while making the most of the existing infrastructure of the time division multiple access system, it is required to provide cellular phones that have communication functions for both systems and that are operable in multiple modes and multibands.
For example, JP 2004-040322A discloses a front end section that performs input/output of signals of the WCDMA system and input/output of signals of three time division multiple access systems, that is, the global system for mobile communications (GSM), the digital cellular system (DCS) and the personal communications service (PCS).
A smaller size and higher integration are required for the front end part of the front end section of a cellular phone typically has the form of a module. Such a module is called a front end module. A front end module including a switch circuit for switching signals is also called an antenna switch module. In the present patent application, a combination of circuits performing processing of high frequency signals and a substrate for integrating these circuits, including such a front end module, is called a high frequency module. As the substrate in a high frequency module, a layered substrate including a plurality of dielectric layers and a plurality of conductor layers alternately stacked is used, for example.
In the front end section that performs input/output of signals of the WCDMA system and input/output of signals of a plurality of time division multiple access systems as disclosed in JP 2004-040322A, a band-pass filter (BPF) is required for selectively allowing WCDMA reception signals to pass. A BPF that selectively allows WCDMA reception signals to pass will be hereinafter called a WCDMA reception BPF. It is required that the WCDMA reception BPF have performance capabilities that achieve a low power loss and a high resistance to power. A block-type dielectric filter is known as a BPF that satisfies such requirements. However, the block-type dielectric filter is relatively large in dimensions. Consequently, if the block-type dielectric filter and a front end module are mounted as individual components on a substrate of a cellular phone, a large area is occupied by the block-type dielectric filter and it is therefore difficult to achieve smaller dimensions and higher integration of the front end section. To solve this problem, it is possible to mount the block-type dielectric filter on the substrate of the front end module and to thereby include the block-type dielectric filter in the front end module. For this purpose, it is required to reduce the thickness of the block-type dielectric filter. However, it is difficult to reduce the thickness of the block-type dielectric filter because of the operational principle. Therefore, it is also difficult to include the block-type dielectric filter in the front end module.
In the front end section disclosed in JP 2004-040322A, the WCDMA reception BPF and a switch for switching signals other than WCDMA reception signals are respectively connected to an antenna through a phase line so as to allow the front end section to be capable of receiving WCDMA reception signals at all times. The phase line adjusts the impedance of each of the path from the antenna to the WCDMA reception BPF and the path from the antenna to the switch, and thereby separates WCDMA reception signals from other signals. The following problem occurs in the case where, in such a configuration, the WCDMA reception BPF and the front end module are mounted as individual components on the substrate of a cellular phone. In this case, it is required to provide a phase line on the substrate of the cellular phone for adjusting the impedance of the path from the antenna to the WCDMA reception BPF and to adjust the characteristic of the front end section by this phase line. However, this adjustment is difficult. If it is possible to include the WCDMA reception BPF in the front end module, it is made possible to adjust the characteristic of the front end section only by the phase line in the front end module and it is therefore easy to adjust the characteristic. However, as previously described, it is difficult to include the WCDMA reception BPF in the front end module in the case in which a block-type dielectric filter is used as the WCDMA reception BPF.
A surface acoustic filter is known as a filter that can be reduced in size and thickness. However, since the surface acoustic filter has a low resistance to power, it is not suitable for use as a WCDMA reception BPF in the front end section that is capable of receiving WCDMA reception signals at all times and that has such a possibility that a high-power GSM transmission signal passes through the WCDMA reception BPF, as disclosed in JP 2004-040322A.
Furthermore, as disclosed in JP 10-303068A, for example, a layered BPF employing a resonator made of a conductor layer sandwiched between dielectric layers. The BPF disclosed in this publication has such a structure that a resonator electrode is sandwiched between two high-permittivity layers, and two low-permittivity layers are respectively disposed on both sides of the two high-permittivity layers in the direction in which the layers are stacked. A shield electrode is disposed between each of the high-permittivity layers and each of the low-permittivity layers.
JP 5-145308A discloses a dielectric resonator having such a structure that a resonant conductor is sandwiched between two high-dielectric layers, two low-dielectric layers are respectively disposed on both sides of the two high-dielectric layers in the direction in which the layers are stacked, and furthermore, ground (GND) electrodes are respectively disposed on both sides of the two low-dielectric layers in the direction in which the layers are stacked.
JP 5-152803A discloses a dielectric filter having a structure similar to that of the dielectric resonator disclosed in JP 5-145308A.
JP 9-205306A discloses a micro-wave circuit element having such a structure that quarter-wave strip lines are respectively provided on both surfaces of a center dielectric material, two inner dielectric materials are respectively disposed on both sides of the center dielectric material in the direction in which the layers are stacked, two outer dielectric materials are respectively disposed on both sides of the two inner dielectric materials in the direction in which the layers are stacked, and ground electrodes are further disposed respectively on both sides of the two outer dielectric materials in the direction in which the layers are stacked.
An electromagnetic shield is required for a layered BPF to prevent influences of external electromagnetic fields. The shield electrode of JP 10-303068A, the ground electrodes of JP 5-145308A and JP 5-152803A , and the ground electrode of JP 9-205306A each have the function of a shield.
For a layered BPF, it is effective to dispose a high-permittivity layer around a resonator to achieve a reduction in size. A high-permittivity layer is disposed around the center conductor in the structure disclosed in each of JP 5-145308A, JP 5-152803A and JP 9-205306A, too.
In the front end section performing input/output of signals of the WCDMA system and input/output of signals of a plurality of time division multiple access systems, it is possible to employ the above-mentioned layered BPF as the WCDMA reception BPF. However, the following problem occurs in this case. As previously described, a shield is required for the layered BPF. In addition, it is effective for the layered BPF to dispose a high-permittivity layer around a resonator to achieve a reduction in size, as previously described. In the layered BPF having such a structure, since the high-permittivity layer is disposed between the resonator and the shield, it is likely that a high capacitance is generated between the resonator and the shield. As a result, the Q of the resonator is likely to decrease, as disclosed in JP 5-145308A. To prevent this, it is required to increase the distance between the resonator and the shield. However, this increase in distance leads to an increase in thickness of the entire layered BPF, and if this layered BPF is mounted on a substrate, the thickness of the entire layered structure including the substrate and the BPF is increased. It is therefore difficult to downsize the front end section.